Method of coating zirconium



United States Patent 3,076,733 METHOD OF COATING ZIRCONIUM George F. Otto, Oreland, Pa., assignor to Amchem Products, Inc., Ambler, Pa'., a corporation of Delaware No Drawing. Filed May 3, 1960, Ser. No. 26,4M 16 Claims. (Cl. 1486.15)

This invention relates to the art of coating zirconium or alloys thereof in which zirconium is the principal ingredient and it has particular reference to the provision of a method for applying a chemically bonded coating on zirconium surfaces. In the following disclosure as well as in the appended claims the word zirconium will be used as applying ,both to the pure metal as well as to alloys thereof wherein zirconium is the principal ingredient.

As is well known zirconium is a metal with which it is difficult to work. It is not readily attacked by most acids and alkalies and has a tendency to gall rather erely when in frictional contact with other metal surfaces or when subjected to mechanical cold-working operations.

Heretofore various processes have been proposed for applying a chemically bonded coating to zirconium surfaces and, generally speaking, these prior art processes are based upon the use of coating solutions in which the coating producing agents are selected from the class consisting of alkali metal, phosphate and halide ions plus accelerating agents. The surfaces are subjected to the action of such solutions and while the coating baths have proven to be satisfactory as initially formulated it has been observed that the bath activity and its coating action tends to diminish rather rapidly with bath age. This difliculty is particularly true after the coating ingredients of the bath have been replenished several times. The problem is believed to be caused by a continuous build-up of alkali metal ions in the solution of the bath. For instance, as the alkali metal ions accumulate it has been observed that there is a salting out and a sludging of the essential coating producing ingredients.

Previous attempts to overcome the difiiculties just mentioned have not been wholly successful because a satisfactory method for removing a portion of the build-up of alkali metal ions has not as yet been found and for this reason it has been necessary to discard large portions of a used bath and to add equivalently large portions of new bath materials in order to insure continuously satisfactory coating operations. Obviously, this procedure involves waste of materials and while partially successful it appreciably increases the cost of maintaining the process in operation.

The principal object of the present invention is to overcome the prior art difiiculties above mentioned and I attain this objective by employing an all-acid coating solution or bath with the complete exclusion of alkali metal ions.

A further object of the invention is the provision of a process of coating zirconium by means of which it is possible to produce a tightly adherent, chemically bonded coating not only when the bath is initially prepared but also after the bath has been extensively used in the coating of a long succession of zirconium surfaces.

The invention is based upon the discovery that a chemically bonded coating of excellent quality may be produced on zirconium surfaces by subjecting the surface to the action of an aqueous acid solution containing as its principal and essential coating producing ingredients both phosphoric and fluoboric acids.

In the following disclosure it is to be understood that reference to phosphoric acid means the 85% (weight/volume) grade and reference to fluoboric acid means the 50% aqueous grade unless otherwise specified.

With respect to the amounts of essential coating producing ingredients to be used in the treating solution I have found that the ratio of phosphoric to fluoboric acids should lie within the range of about 0.5 :1 to about 10:1. Moreover, I have found that the total amount of both phosphoric and fiuoboric acids should be within the range of from 10 to 150 ml./liter of solution. Use of a ratio of phosphoric2fluoboric acids greater than 10:1 produces no useful coating on zirconium surfaces, if indeed a coating is produced at all, while use of a ratio less than 0.5 :1

- results in a bath having too great an activity so that severe etching of the zirconium articles results. The optimum and preferred ratio of phosphoric to fluoboric acid has been found to be about 2:1. Use of less than 10 mls. of total acids/liter of solution yields a bath having a rate of reaction which is too slow to be practical, while use of more than mls. of total acids/liter is not preferred due to increased bath activity and concentration which results in formation of modified coatings inferior to those of the invention.

1 have found that the pH of the coating bath generally falls within 0.6 to 2.0 although this is not too critical. Below a pH of about 0.6 the solution tends to exert an etching action on zirconium metal which results in unnecessary waste of metal, while above a pH of about 2.0 little or no coating may be produced on the articles being treated. The preferred pH has been found to lie in the range of about 0.6 to 1.45.

As noted above, the principal and essential coating producing ingredients are phosphoric and fluoboric acids. It has been found that in order to insure continuous satisfactory operation of the coating baths of this invention these acids per so must be used. Use of soluble salts of phosphoric or fluoboric acids will not sutfice for the purpose of this invention. use of boric acid with alkali metal or ammonium fluorides or bifiuorides, or use of hydrofluoric or fiuosilicic acids, in conjunction with phosphoric acid or a soluble phosphate salt, will not produce satisfactory coatings on zirconium articles. The use of metallic cations, except as noted below, should also be avoided in order to insure the most satisfactory coating production as well as the prevention of sludging or salting out from the bath of essen tial coating producing ingredients. Similarly, foreign anions are to be avoided inasmuch as these tend to have a poisoning or inhibiting effect upon solution activity. For example, the addition of fluoride or nitrate ions, per se to the coating solutions of this invention may result in detrimental effects on subsequent coating formation.

The coating solutions of this invention may be applied to the work in any manner which is capable of effecting continuous contact of the solution with the zirconium surface. The usual and optimum methods are immersion and spraying. However, before attempting to carry out the process of this invention, it is preferable that the surface of the article to be coated be reasonably clean. Although the cleaning step forms no part of the present invention it may be accomplished by acid, alkaline or solvent agents, all of which methods are familiar in the art.

So far as the bath temperature is concerned com pletely satisfactory coatings have been obtained while op- Patented Feb. 5, 1963 a I have also discovered that of 30 seconds'to 10 minutes at room temperature, it has 1 been found that prolonged contact of the zirconium surface with the baths of this invention tends to redissolve the protective coating. This phenomenon is particularly evident at-elevated temperatures, so that in the interest-s of obtaining the optimum coating protection I prefer to operate the'process at a temperature ranging from average room temperature to about 100 F. for a period of about two'to five minutes.

While not essential to the production of completely satisfactory coatings, I have made an additional discovery, namely, that zirconium surfaces which have been coated according to my invention as above set forth can often be still further improved if the coated surface is subjected to the action of certain hot alkaline solutions as described just below. The choice of alkaline agents suitable for-this purpose appears to be critical and is limited tophosphate, silicates and mixtures thereof in. aqueous solutions at concentrations from 5 to 50 grams/liter. A Typical examples of suitable alkaline agents falling withinthisclass include the meta and the ortho alkali silicates and trisodium phosphate. Within the concentration range just mentioned solutions of these agents will have a pH of 10.0 to 12.5 and their effect is best obtained by immersing the coated zirconium surface in the alkaline bath. 3

However, before applying the alkalinebath treatment the coated zirconium surface should first be.rinsed with water in order toremove all traces of any acid residues which may 'be-present.from the coating operation. After rinsing, the surfaces may be, treated with the alkaline solution referred, to atvtemperatures ranging from average livingor room temperatures, i.e., about 70 up to as much as 180 F., for a period of time ranging from one to five minutes. In practice I have found that excellent results aresecured by employing an alkaline so-.

lution for the purpose described which consists of trisodium phosphateat a pH of from about 11 to about 12 and for 'a period of time of approximately one minute at a temperature in the neighborhood of 100 F. Be-

cause of the excellent results so secured I prefer this.

type of alkaline treatment for the coatings of my invenconcentration begins to demonstrate an inhibiting action with respect to coating formation. The preferred method of adding these ions is in the form of their oxides, i.e., SnO and M00 However, use of their respective chlo ride salts has also been found to be acceptable.

By way of example and with no intent to be limited thereto, the following coating compositions are presented as typically representative of this invention.

Water, to make 1 liter;

Examples I.t0.III, above,-.are suitable for'use in coating zirconium without further change.

Frequently, it. is desirable toprepare a..concentrated admixture of chemical constituents for use in preparing fresh coating solutions or for replenishment of same. A

typical admixturefalling under this embodiment is shown I in Example IV below- Example IV Percent H PO 85% 40.0 H81 50% 40.0 Water 20.0

Typical examples of accelerated zirconium coating solu-.

tion. However,.as will be understood by those skilled in the art, use of longer treatment cycles will accompany the use of more dilute alkaline solutions and/or use of lower treatment temperatures. For this reason, there is no'lhard and fast rule eitheras to temperatures or length of treatment. The purpose. of the treatment is to appreciably harden the coating and this hardening can be attained in greater or less degree by use of alkaline treatments. where the solution may vary considerably within the broad temperature range given and where the length of trea'tmentmay vary with the range given, namely, from one to five minutes.

While I have, indicated above that the use of metallic cations in my coating solutions should be avoided I have found that the addition of a small amount of stannous and/or molybdenum ions to the coating baths of my in vention results in the production of a coating which is surprisingly, heavier and more crystalline. I cannot explain just why this is, but I have discovered that the use of from .0.0ltov0.2% weight/volume (i.e. grams/ 100 mls.) of stannous or. molybdenum ion when added to a freshly prepared coating bath yields the heavier and more crystalline coatingsreferred to. Use of less than 0.01%

of the metal accelerator ions does not exhibit this unexpected property, while use of more than 0.2% of the accelerator does not appear to offer any additional advantage over the results obtained from the indicated range, and .at least with respect to molybdenum ion I have found that use of amountsjgreater than about 0.5%

7 Example V H3PO4, ml HBF 50% "mi... 15.0 SnO grams" 1.0 Water, to make 1 liter.

Example VI Percent H PO 85% 40.0 HBF 50% 40.0 M00 0.5 Water 19.5

shown in Examples V. andVI below.

Example V- is suitable'for use, as formulated above, while Example VI represents an accelerated concentrated formulation similar to Example IV. V I

Illustrative but non-limiting examples of alkaline, coating-treating solutions falling under the purview of this invention are as follows:

Example VII Grams Trisodium phosphate (Na PO .12H O) 10 Water, to make 1 liter.

As previously indicated, the coatingv produced in .accordance with my invention imparts, a tightly adherent protective layer on zirconium surfaces. This coatiugacts as an excellent parting material on zirconium when used in frictional contact with other metals or with other zirconium surfaces. If desired, the coatings of my invention may be lubricated with common lubricants familiar in the art so that the advantages of such lubricants may also be obtained.

I claim:

1. The method of coating zirconium which comprises treating the surface with a solution consisting essentially of phosphoric acid, fluoboric acid and Water, the ratio of phosphoric to fluoboric acid being from about 0.5:1 to about :1 and the total of both acids being from about 10 to 150 ml./liter of solution.

2. The method of claim 1 wherein the ratio of phosphoric to fluoboric acid is about 2:1.

3. The method of claim 2 wherein the pH of the solution lies between 0.6 and 2.0.

4. The method of claim 1 wherein the pH of the solution lies between 0.6 and 2.0.

5. The method of claim 1 wherein the pH of the solution lies between 0.6 and 1.45.

6. The method of coating zirconium which comprises treating the surface with a solution consisting essentially of phosphoric acid, fluoboric acid and water, the ratio of phosphoric to fluoboric acid being from about 0.521 to about 10:1 and the total of both acids being from about 10 to 150 ml./ liter of solution, rinsing the treated surface to remove acid residues and then subjecting the rinsed surface to the action of an alkaline solution consisting essentially of phosphates, silicates and mixtures thereof, said alkaline solution being employed at a temperature between 70 and 180 F.

7. The method of claim 6 wherein the concentration of the materials in the said alkaline solution is from 5 to 50 grams/liter.

8. The method of claim 7 wherein the alkaline solution is employed for a period of time of from one to five minutes.

9. The method of claim 7 wherein the alkaline solution is a solution of trisodium phosphate at a pH of 11 to 12 and wherein the treatment with said alkaline solution is continued for about one minute at about 100 F.

10. The method of claim 1 wherein the solution also contains, as an addition agent, from 0.01 to 0.2% (-weight/ volume) of metallic ions from the class consisting of stannous and molybdenum ions.

11. The method of coating a succession of zirconium surfaces which comprises treating them with a solution consisting essentially of phosphoric acid, fluoboric acid and water, the ratio of phosphoric to fluoboric acid being from about 0.5 :1 to about 10:1 and the total of both acids being from about 10 to 150 mL/liter of solution, and renewing the solution by periodic additions of said ingredients as required to maintain the solution within the limits specified.

12. The method of claim 11 wherein the solution is used at a temperature between about and 130 F.

13. The method of claim 11 wherein the solution is used at a temperature between about 70 and 130 F. and wherein the ratio of phosphoric to fluoboric acid is about 2:1.

14. The method of claim 11 wherein the solution is used at a temperature between about 70 and 130 F. and wherein the pH of the solution lies between 0.6 and 2.0.

15. The method of claim 11 wherein the solution is used at a temperature between about 70 and 130 F., wherein the ratio of phosphoric to fluoboric acid is about 2:1 and wherein the pH of the solution is kept between 0.6 and 1.45.

16. The method of claim 11 wherein the solution is used at a temperature between about 70 and F.

and wherein the treatment is continued for a period of from about two to five minutes.

References Cited in the file of this patent UNITED STATES PATENTS 1,555,798 Gravell Sept. 29, 1925 2,477,841 Ward Aug. 2, 1949 2,479,564 Gilbert Aug. 23, 1949 2,839,439 Stapleton June 17, 1958 2,864,732 Miller et a1. Dec. 16, 1958 

6. THE METHOD OF COATING ZIRCONIUM WHICH COMPRISES TREATING THE SURFACE WITH A SOLUTION CONSISTING ESSENTIALLY OF PHOSPHORIC ACID, FLUOBORIC ACID AND WATER, THE RATIO OF PHOSPHORIC TO FLUOBORIC ACID BEING FROM ABOUT 0.5:1 TO ABOUT 10:1 AND THE TOTAL OF BOTH ACIDS BENG FROM 0.5:1 TO 10 TO 150 ML./LITER OF SOLUTION, RINSING THE TREATED SURFACE TO REMOVE ACID RESIDUES AND THEN SUBJECTING THE RINSED SURFACE TO THE ACTION OF AN ALKALINE SOLUTION CONSISTING ESSENTIALLY OF PHOSPHATES, SILICATES AND MIXTURES THEREOF, SAID ALKALINE SOLUTION BEING EMPLOYED AT A TEMPERATURE BETWEEN 70* AND 180* F. 